1. Field of the Invention
This invention relates to polyurethane hot melt adhesives that are moisture-curable and contain reactive amine catalysts. The invention also comprehends a method for accelerated moisture-curing of polyurethane reactive hot melt adhesives using one or more catalysts that contain active hydrogen.
2. Background Art
Traditionally, reactive hot melts are considered to be the most completely solid, solvent-free, single component urethane prepolymers. Conventional hot melts can metamorphose from a solid to a liquid state. In contrast, reactive hot melts undergo an irreversible chemical reaction (xe2x80x9cthermosettingxe2x80x9d) once exposed to ambient moisture.
Reactive polyurethane hot melt adhesives are solvent-free polyurethane prepolymers end-capped with isocyanate groups which are solid at ambient temperatures. Such adhesives are melted or liquidified by heating and then applied to a substrate. During and particularly after application, the isocyanate groups react with water molecules from ambient moisture as well as in the substrate itself to form a thermoset. Such thermosets exhibit adhesive properties superior to those obtained from conventional hot melts which do not contain reactive isocyanate groups.
Even though the initial or xe2x80x9cgreenxe2x80x9d strength of reactive polyurethane hot melt adhesives is sufficiently high that the bonded substrates can be readily handled, typical moisture curing of such adhesives takes days to fully accomplish without a catalyst. It is desirable to accelerate curing of the adhesive, especially in the first day or so after application so that the bonded parts could be easily shipped and delivered shortly after the bond is created. However, conventional polyurethane catalysts such as those based on heavy metals (e.g., tin) may compromise the thermal stability of the adhesive.
The use of 2,2xe2x80x2-dimorpholinodiethyl ether (DMDEE) and similar morpholino-containing substances as catalysts in reactive polyurethane hot melt adhesives is known from U.S. Pat. Nos. 5,550,191 and 6,020,429 (incorporated herein by reference in their entirety). It is said that catalysts of this type could speed up the moisture cure without significantly affecting thermal stability. However, the incorporation of such catalysts provides adhesives which are not free of volatile organic compounds (VOCs). Indeed, DMDEE is reported by Huntsman Corporation to contain about 76% VOCs. This determination has been made using ASTM Protocol D-2369, also known as EPA Method 24. This testing protocol measures the percentage weight loss after heating in an oven at 100xc2x0 C. for one hour. It is known that DMDEE can be used as a catalyst for moisture curing of liquid, one component polyurethane adhesives during applications at ambient temperature, as DMDEE evaporates slowly at ambient temperature. During application at elevated temperatures, such fugitive catalysts can escape from the adhesive. In addition to the environmental problems thereby created, the loss of catalyst will make the curing rate less consistent than would be desirable. This will be particularly pronounced in open roll coater applications where the adhesives are constantly exposed to high temperatures (e.g., 121xc2x0 C.) and a strong, open ventilation air flow. With different exposure times, different DMDEE residues in the adhesives will result in a curing rate that is dependent on exposure time.
WO 97/19122 describes a one-component reactive polyurethane adhesive containing catalysts having at least one tertiary amine and at least one functional group such as xe2x80x94OH capable of being incorporated into the polyurethane. This reference lists suitable catalysts and mentions hot melt adhesives. However, the catalyst is used at a high concentration (at least 1%), and teaches that lower concentrations will not provide satisfactory reactivity. At concentrations above 1%, however, reactive hot melts tend to exhibit poor stability.
The invention includes a catalyzed reactive hot melt adhesive that comprises the reaction product of a reactive polyurethane adhesive and a catalytically effective amount of at least one amine non-fugitive catalyst with at least one active hydrogen, a method for reducing the curing time, and a method for making it.
The disclosed hot melt adhesive circumvents the afore-mentioned problems in that it uses a reacted-in catalyst system based on one or more tertiary amine catalysts containing at least one active hydrogen (i.e., at least one functional group capable of reacting with the isocyanate groups present in the polyurethane prepolymer). Such a reaction leads to incorporation of the catalyst into the polyurethane and thermoset adhesive derived therefrom, rendering the catalyst non-volatile.
Adhesive systems which are VOC-free and yet still exhibit accelerated curing rates are thus provided. Moisture cure rate is still desirably accelerated, especially in the first 24 hours. Thermal stability of the adhesive is not significantly affected. More consistent curing is possible as compared to DMDEE-catalyzed systems since the catalyst, once reacted, is not volatile and thus has a greatly reduced tendency to escape from the adhesive during application.
The invention discloses a catalyzed reactive hot melt adhesive and a method for increasing the moisture curing rate of the reactive hot melt adhesive. The adhesive includes the reaction product of a polyurethane adhesive and a catalytically effective (as defined later herein) amount of at least one amine catalyst containing at least one active hydrogen. The curing speed is achieved by incorporating about 0.005 to about 0.5% by weight of the amine catalyst.
Preferably, the active hydrogen-containing functional group is an hydroxyl (xe2x80x94OH) group. In one desirable embodiment of the invention, the hydroxyl group is present as part of an alkanol group attached to a tertiary amine or ether oxygen. In desirable embodiments of the invention, the catalyst does not contain any primary or secondary amine groups. Preferably, at least two tertiary amine groups are present. Mixtures of different catalysts can be utilized. Suitable catalysts for purposes of the present invention include the following substances:
N,N,Nxe2x80x2-trimethyl-N-hydroxyethyl-bisaminoethyl ether (commercially available as JEFFCAT ZF-10 from Huntsman Corp.)
N,N-bis (3-dimethylaminopropyl)-N-isopropanol amine (commercially available as JEFFCAT ZR-50 from Huntsman Corp.)
N-(3-dimethylaminopropyl)-N,N-diisopropanolamine (commercially available as JEFFCAT DPA from Huntsman Corp.)
N,N-dimethylethanolamine (commercially available as JEFFCAT DMEA from Huntsman Corp.)
2-(2-dimethylaminoethoxy) ethanol (commercially available as JEFFCAT ZR-70 from Huntsman Corp.)
The catalyst is used in effective amounts, i.e., an amount effective to catalyze the desired moisture cure of the polyurethane prepolymer. Generally, the amounts vary within the range of about 0.005 to about 0.5% by weight, preferably 0.005 to about 0.1% by weight. The optimum amount depends upon the isocyanate content of the prepolymer, the desired cure rate, temperature, moisture level, and other factors.
The other components of the reactive polyurethane hot melt adhesives may be any of the substances known in the field, such as those described in more detail in U.S. Pat. Nos. 5,550,191 and 6,020,429 (incorporated herein by reference in their entirety).
Experiments were conducted using a commercial reactive polyurethane hot melt adhesive based on a polyether-polyester-MDI-based polyurethane, known as R243, which is available from Henkel Corporation. This adhesive (xe2x80x9cFormulation 1xe2x80x9d) contains no catalyst. The curing profile and viscosity increase (expressed as viscosity divided by starting viscosity) of R243 with 0.02 wt % DMDEE catalyst added (xe2x80x9cFormulation 2xe2x80x9d) and R243 with 0.02 wt % N,N,Nxe2x80x2-trimethyl-Nxe2x80x2-hydroxyethylbisaminoethyl ether added (inventive xe2x80x9cFormulation 3xe2x80x9d) were also evaluated. The viscosity increase of a satisfactory catalyzed reactive hot melt adhesive should approximate the viscosity increase of the uncatalyzed system in order to achieve good thermal stability.
Minor differences in the viscosity increase (which are indicative of thermal stability) of the three formulations were thus observed when the adhesive formulations were exposed to an open cell (Brookfield system) at 121xc2x0 C. The catalyzed systems, Formulations 2 and 3, showed a faster viscosity increase compared to the uncatalyzed system, Formulation 1, which is expected. Formulation 3 also showed a slightly faster viscosity increase compared to that of Formulation 2, which could be attributed to a higher catalysis efficiency in Formulation 3 (see Tables II and III).
However, the three formulations exhibited significantly different curing profiles, as shown in Tables II and III below:
The third formulation in accordance with the present invention exhibited much faster curing than the first formulation (as shown in Tables II and III) within the first 24 hours, especially during the first 6 hours. This is demonstrated by mechanical test (Instron) results, such as higher ultimate tensile strength (Table II) and higher elongation at break (Table III). The second formulation also exhibited an accelerated curing rate (as shown in Tables II and III), but to a lesser degree than the third formulation, especially during the first 4 hours or so.